Ausforming of NiTi

Lattice defects are introduced into stable austenite () by hot rolling up to 70% (in one pass) in a temperature range between 900 > TAF > 340 °C (ausforming (AF)). The changes in microstructure, thermal transformation behavior, as well as normal and anomalous properties in stress-, strain-, and temperature-space were studied. Martensitic transformation temperatures are lowered with decreasing deformation temperature and increasing amount of deformation. A two-stage reaction is induced at TAF 400 °C. Deformation twins are the predominant microstructural feature, in addition to dislocations at TAF 700 °C. Highly elongated grains indicate that long-range recrystallization is absent at all ausforming temperatures. Semicoherent pecipitation in addition to high dislocation densities are likely to be responsible for the premartensitic anomalies of thermal transformation for TAF 400 °C. Ausforming leads to a considerable increase in conventional strength properties (yield stress, tensile strength, elongation) without loss of transformability and, consequently, shape memory or pseudo-elasticity.     

Effects of ausforming on isothermal bainite transformation behaviour and microstructural refinement in medium-carbon Si–Al-rich alloy steel

The effects of ausforming temperature, strain and strain rate on isothermal bainite transformation behaviour and microstructural refinement in medium-carbon Si–Al-rich alloy steel were determined by thermomechanical simulation. Results show that ausforming decreased the martensite start temperature and enabled isothermal transformation at low temperatures, resulting in the formation of nanostructured bainite. Decreasing the ausforming temperature and increasing the ausforming strain did not only reduce the incubation of isothermal transformation but also refined bainite lath and enhanced the hardness of the alloy steel. Strain rate showed a weak influence on the transformation behaviour and microstructure of the obtained nanostructured bainite.     

A dislocation based material model for warm forming simulation

Based on experimental results, a dislocation material model describing the influence of dynamic strain aging on the deformation behavior at elevated temperatures is presented. One and two stage loading tests at different temperatures were performed in order to describe the hardening behavior during the deformation at elevated temperatures as well as the hardening behavior after the dynamic strain aging process. Bergström’s theory of work hardening was used as a basis for the model development. In the proposed model a relationship between material coefficients of the classical Bergström model and temperature was investigated. The aim of the new material model was to introduce the least possible amount of new parameters as well as to facilitate the mathematical determination of parameters during the fitting of the model with the experimental data. The developed model was implemented in an in-house FE-Code in order to simulate the material behavior due to the dynamic strain aging. Representative simulation results were compared with the experimental data in order to validate the efficiency and the application range of the model. Simulation of the forming process provided data for optimizing strength properties and enabled process control.     

Critical ausforming temperature to promote isothermal bainitic transformation in prior-deformed austenite

The effects of deformation temperature on phase transformation and microstructure in nanostructured bainite steel were studied. The results indicate that the deformed austenite with a strain of 0.3 at 300°C presents accelerated kinetics of bainitic transformation. However, the amount of bainite in ausformed austenite then reduces with the increase in deformation temperature. A critical deformation temperature, determining whether the bainitic transformation can be promoted, was found in deformed austenite. In addition, the thickness of bainite plate in deformed austenite reduces with the decrease in ausforming temperature. The adjacent bainite ferrite plates grow up interactively, and the intersection angle is about 60–73°. A lower ausforming temperature contributes to a more serious cross-growth phenomenon of bainite plates.     

Effects of DC Current on the Mechanical Behavior of AlMg1SiCu

The effect of an electron wind on the mechanical properties of aluminum is investigated with the ultimate goal of establishing a technique by which the mechanical energy associated with the deformation of a material can be reduced without requiring a significant increase in the material's temperature. In the study presented herein, the effect that the electrical flow has on the mechanical properties of aluminum is examined through tensile testing. However, as electricity is passed through the material, some incidental resistive heating occurs. Therefore, in order to isolate the effect of the electrical flow from that resulting from resistive heat, the effect of transient temperatures on the stress-strain behavior of aluminum is also considered, with, and without, the presence of the electrical flow. In addition, variation in the electrical effect, with respect to the aluminum's temper and electrical pre-treating, is investigated. The experimental results indicate that the electrical current has the potential to substantially reduce deformation energies without causing significant increases in the workpiece temperature. The data also indicates that this effect exists regardless of the temper on the material. Finally, the study found that electrically pre-treating the aluminum produced results similar to that of an annealing process (i.e., the energies were significantly reduced).     

Ausforming of medium carbon steel

The prospect of enhancing the hardness of low alloy steel for the manufacture of fasteners is examined using ausforming, in which the austenite is deformed rapidly at a low temperature to increase its dislocation density before quenching in order to obtain the harder martensite. Surprisingly, small deformations accomplish large gains in hardness and the dislocation density of martensite, with diminishing returns at larger deformations. The main contribution to the hardness has been identified as the extra dislocations inherited by the martensite from the deformed austenite, rather than the refinement of microstructure by the ausforming process. Clear evidence is reported for the mechanical stabilisation of the austenite due to ausforming. Tempering heat treatments tend to diminish the advantages of ausforming.     

Finite element modeling of effective thermomechanical properties of Al–B<Subscript>4</Subscript>C metal matrix composites

The present work aims to investigate the influences of thermal residual stresses and material properties on the thermomechanical deformation behavior of Al–B₄C composites. Boron carbide-reinforced aluminum matrix composites having 4, 8, and 12 vol% boron carbide were fabricated using squeeze liquid stir casting method for experimental characterization of their microstructure, effective elastic moduli and effective CTEs at room temperature as well as elevated temperatures. Next, the thermomechanical behavior of fabricated composites was investigated using finite element modeling. The effects of thermal residual stresses on the effective material properties were examined by simulating the cooling process of MMCs from processing temperature to room temperature. The effective elastic moduli and the effective CTEs were predicted considering linear elastic as well as elastoplastic deformation of aluminum matrix, and the results obtained were compared with the experimental values. The effects of voids on effective material behavior are studied by simulating the void growth and nucleation using Gurson–Tvergaard–Needleman model.     

Experimental measurement and predictive modelling of bending behaviour for viscous unidirectional composite materials

It is widely accepted that the key deformation mechanisms during forming of viscous textile composite (prepreg) sheets are in-plane shear and out-of-plane bending. This paper focuses on the bending deformation mechanism, including experimental characterisation and theoretical modelling of bending behaviour during viscous composite forming. Experimental measurements are obtained by means of a large-displacement buckling test at a variety of displacement rates and temperatures. Some important aspects, such as viscoelastic behaviour, are also investigated. A bending model based on elastic theory combined with uniaxial continuum theory for ideal fibre-reinforced fluids for viscous shear deformation has been developed, using material parameters obtained from industrial manufacturers as input data, such as composite geometry, fibre properties, fibre volume fraction and matrix rheology. Model predictions demonstrate that the model can capture the main characteristics of material properties, such as rate dependence. This bending model can be used in formability analysis for viscous unidirectional composite materials, and might be applied in a finite element forming simulation to account for the bending stiffness.     

Heat treating of a ring rolled Ni–Fe–Cr superalloy

Abstract

Haynes HR-120 alloy (UNS N08120) is a nickel–iron–chromium alloy that exhibits high strength at elevated temperature and resistance to carburising and sulphidising environments. These properties make this alloy suitable for the production of components of land based turbines, including rings. Manufacture and heat treating of such rings require strict control of the processing variables, such as temperature and deformation ratios, as well as the time and temperature of the solution treatment, due to their effect on microstructure and mechanical properties. It is common practice to treat this alloy at temperatures above 1100°C to promote dissolution of undesired particles and recrystallisation of deformed structures, but it has been found that grain coarsening can occur during treatment. The present work presents the results of a series of solution heat treatments that were performed within a broad range of temperatures on industrial ring rolled pieces. It was found that the increment in time and temperature enhances the dissolution of intergranular precipitates that result in the improvement of mechanical properties, but grain coarsening is observed to occur when the material is treated for long times and high temperatures. The best combination of mechanical properties and grain size was obtained by treating the material for half an hour at 1050°C.     

Tensile behaviour of Ni3Al+B intermetallic compound: influence of cold deformation and annealing

Nickel aluminide, intermetallic compound Ni₃Al, is a promising structural material on account of its high strength at elevated temperatures. The influence of cold deformation on the tensile behaviour of an Ni₃Al alloy containing zirconium and boron is presented. The undeformed material, in the as-cast condition, was subjected to varying levels of cold deformation ranging from 11.4%–61.4%, and tensile tests performed. The tensile properties and fracture behaviour of the cold-deformed material are compared with undeformed material to highlight the influence of cold deformation on strength, ductility and fracture behaviour. Tensile tests were performed on cold-deformed plus annealed samples and properties compared with the cold-deformed counterpart in order to elucidate the influence of annealing on tensile behaviour. The intrinsic effects of cold deformation and annealing on microstructure, tensile properties and fracture behaviour are highlighted.     

难变形材料热静液挤压复合精密塑性成形工艺

目的 研究热静液挤压及其复合塑性变形工艺在高密度钨合金、钨铜合金、钛基复合材料及镁合金薄壁细管等难变形材料方面的制备。方法 通过对高密度钨合金难变形材料进行热静液挤压及旋转锻造等塑性成形,分析了材料在成形过程中的微观组织及性能变化规律和强化机制,制备出大长径比穿甲弹弹芯材料。在此基础上,将该复合塑性变形技术拓展至两相不互溶材料钨铜合金、钛基复合材料及大长径比镁合金毛细管等难变形材料方面的制备。结果 热静液挤压及其复合塑性变形工艺在粉末冶金难变形材料的致密化方面具有显著优势,获得材料不仅致密度高,而且有效实现了控形控性;对于镁合金薄壁细管成形而言,也可以实现组织与性能的有效调配,同时材料的精度较高。结论 热静液挤压及其复合塑性变形工艺在难变形材料的制备与成形方面具有独特的优势与广阔的应用前景。     

复合材料T型整体化结构固化翘曲变形模拟

针对复合材料T型整体化结构固化成型的工艺过程,分析了结构经固化而导致翘曲变形的原因;建立了整体化结构翘曲变形预测的理论模型及分析方法; 运用有限单元法计算了T型结构件的内部温度和固化度的分布,以及由于内部化学反应放热、固化引起的体积收缩和材料各个方向热膨胀系数的不一致而导致的结构翘曲变形量,同时考虑了树脂在固化过程中材料参数随着固化度的变化而变化;并研究了翘曲变形与T型结构件尺寸之间的关系。研究表明,选择合适的角材高度、宽度以及倒角半径可以有效地降低结构的翘曲变形。     

Kinetic spraying deposition behavior of bulk amorphous NiTiZrSiSn feedstock

A kinetic spraying process, which is basically a solid-state deposition process, was used for the formation of a fully amorphous coating. By using a pre-heating system for the powder carrier gas and using helium for the process gas, it was possible to form an amorphous coating. The main process parameters evaluated during this study were gas species [N₂ and He] and pre-heating temperature [RT (below T_g) and 550 °C (liquid metallic region)]. Aside from the empirical approach, in-flight particle velocity within the kinetic spraying process was measured using a SprayWatch-2i system. The deposition behavior of a NiTiZrSiSn bulk amorphous powder was observed when it was sprayed using the kinetic spraying process. In order to predict the temperature-dependent deformation behavior of the bulk amorphous material during impact, Vickers microhardness, as an indirect method, was measured at various temperatures.While the bulk amorphous feedstock material was being coated, both the kinetic and thermal energies of the in-flight particles were important. The former affected the deposition of the bulk amorphous coating, while the latter had more effect on the mechanical properties of the coating. Particle deposition behavior was considered from the viewpoint of the environmental effect, such as particle–energy combination, on the deposition behavior. The bonding of the impacting NiTiZrSiSn bulk amorphous particle was primarily caused by temperature-dependent deformation and fracture (local liquid formation) behavior.     

Macro‐ and Nanomechanical Properties and Strain Rate Sensitivity of Accumulative Roll Bonded and Equal Channel Angular Pressed Ultrafine‐Grained Materials

Several processes of severe plastic deformation are suitable for the production of materials with ultrafine‐grained microstructures which are known to exhibit high strength and often good ductility as well as strain rate sensitive behavior. The most promising ones are equal channel angular pressing (ECAP) for bulk material and accumulative roll bonding (ARB) for the production of sheet material. In order to evaluate the influence of the process on these mechanical properties and the strain rate sensitivity, tensile tests, and nanoindentation tests were performed on material produced up to similar effective plastic strains of ε_ARB = 6.4 and ε_ECAP = 6.3. It could be shown that the macroscopic strength is slightly higher for ARB than for ECAP material and vice versa in nanoindentation. Independent of the testing method, the strain rate sensitivities and activation volumes are similar for both materials. Thus, both processes performed up to similar effective plastic strains lead to comparable improvements in the mechanical properties. Additionally it could be shown, that this comparison allows the identification of the dominant deformation mechanism which is responsible for the observed strain rate sensitivity.     

Collapse of continuous fibre composite beams at elevated temperatures

Short beam tests were performed on continuous graphite fibre/thermoplastic matrix composites between 20 and 300°C. At low temperatures collapse and fracture of the beam is initiated by interlaminar shear and delamination. At high temperatures large-scale inelastic deformation was observed, similar to the deformation of an ideal fibre-reinforced material (IFRM) previously described by Spencer, Pipkin and Rogers and co-workers. With dead-weight loading at loads below the collapse load, creep is observed, also involving IFRM-type deformation, leading to time-dependent collapse of the beam.     

基材属性对铝蜂窝共面力学性能的影响

目的研究不同属性的基体材料对铝蜂窝共面压缩力学性能的影响。方法在保持正六边形蜂窝结构参数不变的情况下,改变基材属性,基体材料模型分别选择不同应变强化参数的双线性各向同性强化模型和理想弹塑性模型,建立相关可靠的有限元模型并进行大量的模拟计算。获得相应的变形模式和应力-应变曲线,对曲线进一步处理得到蜂窝共面静动态峰应力,并将结果以图表形式展示并分析。结果随着冲击速度的增加,样品依次出现了"X","V","一"字型3种变形模式,基体材料的应变强化效应使变形趋于均匀化;基体材料的应变强化效应显著增加了蜂窝的静态峰应力,对动态峰应力增量的影响可以忽略,对计算数据处理后得到了应变强化参数与动态峰应力的计算公式。结论基材具有强化特性的蜂窝,其共面静态力学性能优于基材为弹性理想塑性材料模型的蜂窝;在利用数值模拟的方法来研究蜂窝结构共面静态力学行为时,需要考虑基体材料的强化效应。     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

Effect of temperature on the strength and conductivity of a deformation processed Cu-20%Fe composite

The high temperature (22–600 °C) properties were evaluated for a Cu-20%Fe composite deformation processed from a powder metallurgy compact. The ultimate tensile strengths decreased with increasing temperature but were appreciably better than those of similarly processed Cu at temperatures up to 450 °C. At 600 °C, the strength of Cu-20%Fe was only slightly better than that of Cu as a result of the pronounced coarsening of the Fe filaments. However, at temperatures of 300 and 450 °C, the strength of Cu-20%Fe is about seven and six times greater, respectively, than that of Cu, as compared to about a two fold advantage at room temperature. Therefore, Cu-20%Fe composites made by deformation processing of powder metallurgy compacts have mechanical properties much superior to those of similarly processed Cu at room temperature and at temperatures up to 450 °C. The pronounced decrease in electrical conductivity of deformation processed Cu-20%Fe as compared to Cu is attributed to the appreciable dissolution of Fe into the Cu matrix which occurred during the fabrication of the starting compacts where temperatures up to 675 °C were used. While the powder metallurgy compacts used for the starting material for deformation processing in this study did not lead to a high conductivity composite, the powder metallurgy approach should still be a viable one if processing temperatures can be reduced further to prevent the dissolution of Fe into the Cu matrix.     

Deformation and fracture of polymer/metal composites subjected to cold rolling

Cold rolling of a sandwich composite with a metallic strip inclusion in a polymeric matrix can produce a range of outcomes, including deformation and fracture of the inclusion. Using different material combinations under the same processing parameters, the results ranged from minor deformation, to folding and/or loss of adhesion, and fracture of the inclusion, with fracture particles varying in size and shape. Comparisons are made between the resulting structures after cold rolling of the polymer/metal composite to geological formations. In particular, the fracture particles obtained resemble rock structures known as boudins. The phenomena of boudinage and folding encountered in the cold rolling of polymer/metal composites is similar to that seen in geology formations although the time and size scale of these events are several orders of magnitude apart. The experimental results reported show that cold rolling applied to a polymer/metal sandwiched composite induces deformation and fracture behaviors that depend on the mechanical properties of the constituents, deformation behavior of the polymeric matrices, interfacial adhesion, and process parameters such as rollers speed and nip-gap.     

Recycling of plastic car components: The case of a multilayer item based on polypropylene

The recycling of a particular internal car component with a multilayer structure such as the dashboard has been extensively investigated. On this item, based on polyolefins, a thermal, spectroscopic, mechanical and morphological analysis has been performed. The recycled material showed a strong worsening in the mechanical and impact properties due to the occurrence of thermo-oxidative and thermo-mechanical degradation phenomena taking place during processing. A limited improvement in the mechanical behaviour has been obtained by adding, during the recycling, a suitable antioxidant and stabiliser additive. A more significant enhancement has been achieved in terms of deformation at break, impact toughness and overall morphology when an ethylene-propylene copolymer and virgin polypropylene were added to the formulation during the recycling process. By this procedure materials with properties suitable to be reused for the same or for similar applications within the car have been obtained.